The electric parameters of voltage, current and power generally describe the varying state of a power line in a distribution system. Voltage and current vary sinusoidally at a generally constant power line frequency of 60 cycles in the United States. Power is a product of the voltage and current. While the amplitudes of voltage and current vary sinusoidally the root mean square (RMS) value of voltage is regulated to nearly a constant value. Current RMS fluctuates widely in response to loads being added or subtracted in the system. The nature of the load can cause the phase angle between the voltage and current to fluctuate and an attempt is made to regulate the phase angle fluctuation within bounds. The instantaneous RMS values of a electric parameter are not particularly useful in determining how hard distribution equipment is being worked. The limit of allowable work for most distribution equipment is generally determined by its heat dissipation ability. Distribution equipment has heat capacity allowing a piece of equipment to absorb energy for later dissipation. The absorption and dissipation of equipment in response to a change in power is exponential in nature. To appropriately monitor how hard distribution equipment is being worked, or the demand level, the metering device must respond to changes in power values exponentially. Given usual conditions, the level of demand is proportional to the demand value of current. In the following discussion, current is primarily used as the example of how demand is monitored.
Demand current is a parameter that utilities want to monitor since it tends to diminish the effects of short duration instantaneous RMS current excursions from the average RMS value of the current caused by switching loads and associated transients. The purpose is to measure current in such a way that the meter's registered value is a measure of load as it affects the heating (and therefore the load-carrying capacity) of the electrical equipment in a distribution system. Often lagged or demand current values are used in conjunction with voltage values to provide demand power. Demand power meters can indicate either real or reactive power or both real and reactive power. Also, of course, it is desirable to know what instantaneous RMS current is at any point in time.
Heretofore electromechanical meters known as thermal demand meters were utilized for determining demand current which is an exponentially averaged current having a selected time constant, showing the effects of the change in instantaneous RMS current as loads are switched on and off a line. The time constant is often selected so that 90% of a step change is registered in 15 miutes. The thermal demand meters use an element which moves when it is heated by current and this movement of the thermal element will approximate the exponential buildup of heat in power distribution equipment. Accordingly, for some time now, thermal demand meters have been utilized since the thermal expansion of the thermal element as it is heated by current causes it to mimic the generally exponential heating and cooling response of power distribution equipment.
Examples of demand meters and how they are utilized by electric utilities are described on pages 458-462 in the Electric Utility Engineering Reference Book entitled Distribution Systems, Vol. III, by Electric Utility Engineers of the Westinghouse Electric Corporation, East Pittsburgh, Pa.
Another demand meter is disclosed in the Sangamo Product Bulletin 10215 Effective April, 1982 entitled "K300 Analyzed Load Meter".
Other systems which make measurements of current, power, volts, amperes and, in some instances, demand current, are also disclosed in the following U.S. Patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 3,505,506 J. Kostalos, Jr. 3,747,068 Bruner et al. 3,911,359 Metcalf 4,041,288 Conway et al. 4,077,061 Johnston et al. 4,125,895 Buhlmann 4,229,795 Vieweg et al 4,236,217 Kennedy 4,253,151 Bouve 4,263,653 Mecklenburg 4,272,816 Matsumoto 4,291,376 McCahill 4,356,553 Steinle et al. Re. 28,706 Dorey ______________________________________